Conclusive evidence has been obtained of the excretion of a glucuronic acid conjugate of N-hydroxy-4-aminobiphenyl following the administration of 4-aminobiphenyl to dogs. This conjugate may be present in the urine in amounts representing as high as 20 percent of the metabolic excretion products of 4-aminobiphenyl. This conjugate apparently functions as the carrier of the N-hydroxy metabolite from its site of formation in the liver to its site of action in the bladder. 2-Napthylamine also forms a similar N-hydroxy conjugate. Attempts to isolate the conjugate of N-hydroxy-4-aminobiphenyl utilizing various chromatographic procedures are continuing. Attempts, thus far unsuccessful, to synthesize this compound are also continuing. Electron microscopic examination of the dog bladder mucosa has revealed the presence of a luminal membrane similar to that observed in the bladder of other species. This membrane is being studied by the freeze-etch technique to determine its composition. Effects of aromatic amine carcinogens on this membrane as observed by electron microscopic and freeze-etch techniques are being studied. The application of the gas chromatograph--mass spectrometer to the identification and quantitation of N-hydroxy and nitroso metabolites of the aromatic amines will be studied.